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Experimental Characterization of the Conversion of Dilute Turbidity-Currents Into Transitional-Flows at the Channel to Lobe Transition


The channel to lobe transition is characterized by scour-pits located immediately downstream of channel confinement. Scour-pits are the product of erosion, and subsequent entrainment of sediment into currents that is carried, and then deposited, within the intermediate to distal reaches of submarine fans. This eroded alluvium is thus incorporated into overriding turbidity-currents, and if the entrained sediments contains a relatively high proportion of silt, it can force these currents to transform into transitional-flows. Such a flow conversion explains the observed ‘hybrid bed’ deposits preserved within the intermediate to distal reaches of submarine lobes. Herein, this study will present the results from a series of laboratory experiments designed to: i) characterize the erosion of an unconsolidated kaolinite bed leading to the formation of scour-pits at the channel (confined) to lobe (unconfined) setting, and ii) determine the volume of clay entrained into overriding dilute turbidity-currents. The spatio-temporal evolution of scour-pit formation was captured by a submerged side-view GoPro and subaerial overhead camera, whilst sediment entrainment from the kaolinite bed was visualized acoustically via a downstream-oriented array of two 0.5 MHz Panametrics V301-SU transducers. Furthermore, local near-bed suspended-sediment concentrations (~ 4-5cm above bed) were acquired using a LISST-Streamwise sampler positioned immediately downstream of the transducer array. This set-up allowed for the direct characterization of the internal structure of relatively dilute turbidity-currents as they enhance their sediment concentrations, and begin transforming into transitional flows. Current evolution was captured using a combination of reflector continuity, amplitude, and mutual coherence, which were calibrated to near-bed suspended-sediment concentrations. Preliminary results show: i) dilute turbidity-currents (less than 0.5% sediment concentration) can increase their near-bed sediment concentrations by > 0.5% through the incorporation of kaolinite via scour-pit erosion, and ii) produced scour-pit geometries are directly linked to sediment supply/availability relative to flow height/width and magnitude.